One of the conventional central systems for cooling and/or heating the interiors of large buildings is a dual duct system. In a dual duct system, there are two channels which supply air cooled to a constant temperature and air heated to variable temperatures respectively. One channel contains a cold deck wherein air is cooled to a temperature below room design temperature. The other contains a hot deck wherein air is heated to a temperature at or above room design temperature.
Air from the cold deck and air from the hot deck are separately distributed to mixing boxes through a cold duct and a hot duct respectively.
A mixing box is a device for mixing varying amounts of each of two streams of air at different temperatures to achieve a constant air output at a desired temperature. There are two air valves in the mixing box, one for each of the air streams, both of which are thermostatically controlled by a room or zone thermostat.
In constant volume systems such as these, the flow of total supply air is not varied. To prevent over-cooling when the space served is not subjected to peak cooling loads, as when the outside temperature is below 95.degree. F, building occupancy is reduced below maximum, and/or equipment utilization is below 100%, hot air provided from the heated air duct is mixed with cold air in suitable proportion to maintain a comfortable temperature in the interior of the building. To accomplish this mixing, each area of the space having an environment to be controlled is provided with one or more dual duct (mixing) terminal boxes which receive air from both the cooled and heated air ducts and, under thermostatic control, mix the heated and cooled air to a suitable temperature and expel it into the controlled environment.
The dual duct constant volume system is inherently inefficient in that energy is required to first cool the air for maximum peak conditions and then to heat the cooled air consistent with the immediate load requirements. Such systems, however, are in common use and, until recently, their inefficiency did not result in inordinate expense due to the heretofore relatively low cost of power. However, recent recognition of a growing shortage of energy resources and an attendant sharp rise in the cost of providing power has made the dual duct constant volume system costly to operate for air conditioning.
It is known in the art today to provide only cooling air to building spaces, temperature control being accomplished by varying the volume of the cooled air supplied to the environment to be controlled. Variable volume terminal boxes capable of regulating air volume flow to an environment under thermostatic control responsive to the environmental temperature are known and such systems are now being built in new construction. There are presently in existence, however, numerous plants which employ the older dual duct constant volume system. To replace such systems with a more modern and efficient single duct, single temperature variable volume system requires great expense and inconvenience.
The scrapping of present ductwork and the expense of purchasing new ductwork as well as the labor costs attending both entail additional expense. Furthermore, the limited capacity of a new single ductwork system where the duct size is confined to that of the former ducts places certain constraints on the energy requirements and means for propelling cooled air through the system. What is therefore needed, is an in expensive and facile method for converting older dual duct constant volume systems to function as single duct constant or variable temperature variable volume systems with a minimum of expense while fully utilizing the total volume capacity of the dual duct system.